In wireless systems, such as digital mobile television systems, wireless local area networks, and wireless asynchronous-transfer-mode (ATM) systems, data signals are superimposed onto a reference carrier signal through signal modulation. Typically, depending upon the mobility and location of the transmitter and receiver, the signal may follow multiple paths from transmitter to receiver. Due to this “multi-path” environment, the receiver may receive multiple copies of the transmitted signal with different transmission delays and different attenuations. As a result, the received signal may suffer interference known as inter-symbol interference (ISI), which limits the channel capacity of a wireless system.
One way of overcoming the ISI is through use of OFDM, which utilizes a plurality of sub-carriers. In an OFDM-based system, a serial data stream is converted into a plurality of parallel data symbols. Each data symbol modulates one of the sub-carriers using a conventional modulation technique, such as binary-phase shift keying (BPSK), and quadrature amplitude modulation (QAM), etc. All of the parallel, modulated sub-carriers are then combined to form an OFDM symbol, and a serial stream of such OFDM symbols is transmitted using an RF carrier signal. A guard interval, or a prefix, of a particular width (typically, a fraction of the OFDM symbol size) may be introduced in the OFDM symbols. The width of a guard interval is chosen such that the overlap (in the time domain) of the OFDM symbols—and, therefore, the ISI—must occur within the guard interval. When the receiver receives the OFDM symbols, the guard interval is removed before the useful data is processed, thereby reducing the effect of ISI. However, a guard interval increases the overall OFDM symbol period, and thus negatively affects the overall bandwidth of an OFDM system.
The sub-carriers in an OFDM system are orthogonal to one another, i.e., they are spaced apart so that an orthogonality exists between every two sub-carriers. Accordingly, in theory, any cross-talk between two sub-carriers (known as inter-carrier interference, or “ICI”) is eliminated. However, the presence of a guard interval in an OFDM symbol, and other amplitude and/or phase distortions caused by, e.g., carrier frequency offset, cause loss of orthogonality between sub-carriers, and as a result, ICI is not eliminated. A conventional solution to this problem is to use a cyclic extension (or a cyclic prefix) of the OFDM symbol as the guard interval to preserve the orthogonality. However, as mentioned above, a guard interval may still reduce the overall bandwidth of an OFDM system.
Another way to eliminate ISI and ICI is to use an equalizer that modifies the amplitude and/or phase of the received data adaptively, for example, by multiplying the received data with a coefficient that includes an amplitude-compensation factor and/or a phase-compensation factor.
Typically, equalization coefficients are determined from the frequency response of the channel, which in turn is related to the channel impulse response. Conventional equalizers process only a specific portion of the channel impulse response to determine the gains of multiple channel paths (generally known as channel taps), which may not completely eliminate noise in the impulse-response data. Such limited processing may result in inaccurate channel frequency response, and therefore erroneous equalization coefficients.